I don't have kids, but I think this might be a generational schism where the default view of older generations is that having kids "is just what you do" without much more thought and younger generations see them as a burden. Obviously, generations of people are not a monolith but I think that holds true generally. You might be right that testerone levels have resulted in a declining birth rate, but you also have to consider methods of birth control that are available to women and how atomized society is at the moment (both sexes can do just fine independently)

There are a lot of variables that are hard to control for.